(a) Technical Field
The present disclosure relates, generally, to a fuel cell system. More particularly, it relates to a purge system for a fuel cell, which can reduce the time required to thaw a frozen valve during cold start-up and reduce the whole start-up time, thus improving the cold start performance.
(b) Background Art
A typical fuel cell system in a hydrogen fuel cell vehicle comprises a fuel cell stack for generating electrical energy by an electrochemical reaction of reactant gases, a hydrogen supply system for supplying hydrogen as a fuel to the fuel cell stack, an air supply system for supplying air containing oxygen as an oxidant required for the electrochemical reaction in the fuel cell stack, a thermal management system (TMS) for removing reaction heat from the fuel cell stack to the outside of the fuel cell system, controlling operation temperature of the fuel cell stack, and performing water management function, and a system controller for controlling overall operation of the fuel cell system.
Preferably, the hydrogen supply system includes a hydrogen tank, high-pressure and low-pressure regulators, a hydrogen valve, a hydrogen recirculation system, etc., and the air supply system includes an air blower, an air valve, a humidifier, etc., and the TMS includes a coolant pump, a radiator, etc.
Preferably, in the hydrogen supply system, high pressure hydrogen supplied from the hydrogen tank sequentially passes through the high-pressure and low-pressure regulators and then is supplied to the fuel cell stack at a low pressure. In the hydrogen recirculation system, a blower provided in a recirculation line recirculates unreacted hydrogen of an anode (“fuel electrode” or “hydrogen electrode”) of the fuel cell stack to the anode, thus recycling the hydrogen.
In the air supply system, dry air supplied by the air blower passes through the humidifier to be humidified by absorbing water from exhaust gas (wet air) discharged from a cathode (“air electrode” or “oxygen electrode”) of the fuel cell stack and then is supplied to the cathode of the fuel cell stack.
An urgent and important consideration of a fuel cell vehicle is to improve cold start performance. When the fuel cell system is exposed to sub-zero temperatures for a long time, water present in various components of the system such as valves as well as the fuel cell stack is frozen, which makes it difficult to start the vehicle.
Water is produced by the electrochemical reaction of reactant gases in a polymer electrolyte membrane fuel cell (PEMFC) applied to the fuel cell vehicle, and thus the water is collected in the fuel cell stack, pipes connected to the fuel cell stack, and various components mounted in the pipes.
Therefore, when the outside air temperature falls below zero, the water present in the fuel cell stack and other components is frozen, and thus an external heat source is required to melt the ice in order to start the vehicle afterwards.
However, when the ice is melted by heat from the external heat source, it takes varying amounts of time to completely thaw the individual components according to the temperature and weight of each component and the amount of water present in each component.
In particular, since it is impossible to operate the fuel cell system before several components that are important for the operation of the fuel cell are thawed, it is very important to thaw the key components as soon as possible during cold start-up in order to reduce the start-up time.
In the case of the valves, an air cut-off valve, a hydrogen recirculation valve, a hydrogen purge valve, etc. are always exposed to a wet environment during operation of the fuel cell, and thus the valves are frozen at a temperature below the freezing point. Accordingly, it is necessary to thaw the valves using a heater during the cold start-up.
For this purpose, a heating system such as a positive temperature coefficient (PTC) heater, which is operated by electric power supplied from a battery, is preferably provided in the valves to thaw the valves to be normally operated during the cold start-up.
FIG. 1 is a schematic diagram of a purge system in which a hydrogen purge valve is suitably installed at an anode outlet of a fuel cell stack.
Preferably, the hydrogen purge valve 12 is an electronic valve, which is opened and closed in response to a control signal of a controller to control the hydrogen concentration in the fuel cell stack 1, through which hydrogen is used to remove impurities such as water, nitrogen, etc. present in the fuel cell stack 1 and to increase hydrogen utilization.
The hydrogen purge valve 12 is in a closed state while the operation of the fuel cell stack 1 is stopped after shut-down and then is opened to exchange the gas in an anode channel to hydrogen during start-up of the vehicle. Unlike a hydrogen supply valve located in a position where dry hydrogen is supplied and operated immediately without a thawing process, the hydrogen purge valve 12 is preferably located at the outlet of the fuel cell stack 1, and thus water is always present in the hydrogen purge valve 12 during operation of the fuel cell system.
Accordingly, when the fuel cell system is exposed to a cold environment, for example during the winter, the water present in the valves is frozen, and the valve is not operated due to ice. Therefore, various methods are used to allow the valves to be freely opened during the cold start-up.
In one exemplary method that allows the valve to be opened at a temperature below the freezing point, a heater configured to surround the valve is used to thaw the valve as mentioned above. In this case, it takes a long time to thaw the ice present in the valve, which leads to an increase in the start-up time.
In another exemplary method, a valve using an impact column is used, in which an impact is suitably applied to the inside of the valve such that the ice is split and melted or discharged to the outside. This method also requires time of about several tens of seconds to open the valve, which causes a delay in the start-up and, especially, reduces the durability of the valve.
Further, another method comprises continuously heating the valve so as not to be frozen, in which a heater surrounding the outside of the valve is continuously operated even after the operation of the fuel cell system is stopped during the winter such that the water present in the valve is not frozen. However, since the electric power of a battery is used to operate the heater, the battery may be discharged when the fuel cell system is exposed to low temperatures for a long time.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.